Dell uses proprietary parts, including and not limited to non-standardized motherboards. Meaning a standard matx or atx most likely won't fit, or if it does fit physically, the standoffs won't line up, or the i/o shield is placed wrong etc.
On top of that, Dell is a big fan of negative pressure systems, no intake fans and often no airflow at all from the front area of the case, so even if the motherboard did actually swap out, you'll be facing very high cpu/gpu temps from lack of airflow.
It's one of those situations where just forgetting about what you currently own and starting from scratch is often the wisest and least frustrating course of action, even if your wallet doesn't agree.